1. Field of the Invention
The present invention relates to a steam-injection type gas turbine system including a steam-injection type gas turbine and a waste heat boiler, in which steam generated by the waste heat boiler is injected into a combustor to increase the output, and is supplied to the stationary blades of the gas turbine to cool the stationary blades.
2. Description of the Related Art
A cogeneration system disclosed in, for example, JP-A No. Hei 6-108877 uses power generated by a gas turbine for power generation, recovers waste heat from the gas turbine and uses the recovered waste heat for covering heat demand, such as air conditioning and hot-water supply. The cogeneration system converts the energy of a single energy source, such as a fuel gas, into effective electrical and thermal energies. Since the temperature of the exhaust gas for raising the inlet temperature of the turbine from another turbine is raised to enhance the output of the turbine, the heat-electric ratio, i.e., the ratio between the thermal energy of the exhaust gas and generated power, increases. Consequently, it occurs sometimes that an amount of steam exceeding an amount of steam necessary for use as process steam by various steam loads is generated when the heat of the exhaust gas discharged from the gas turbine is recovered by a waste heat boiler or the like. Surplus part of steam generated by using recovered waste heat, remaining after the steam is used for thermal demand is injected into combustor for a gas turbine to enhance the output of the gas turbine and to improve the thermal efficiency of the same by reducing the combustion temperature of the combustor by the injected steam and increasing fuel supply rate.
The gas turbine converts the energy, i.e., heat and pressure, of a high-pressure, high-velocity combustion gas jetted by the combustor into kinetic energy to deliver its mechanical output through a rotating shaft. Therefore the stationary blades fixed to the casing of a gas turbine and forming gas-turbine nozzles are exposed directly to the high-temperature combustion gas and hence the stationary blades must be cooled. The conventional gas turbine cools its stationary blades with compressed air supplied by a compressor that supplies compressed air to a combustor. Consequently, the amount of the compressed air supplied to the combustor decreases, the output of the gas turbine decreases accordingly, and the thermal efficiency of the gas turbine decreases.
Incomplete combustion occurs in the combustor if steam is injected into the combustor at an injection rate exceeding a predetermined level. Accordingly, the surplus exhaust gas that does not need to be supplied to the waste-heat boiler or the surplus steam left after using steam for thermal purposes is discharged into the atmosphere. Although the cogeneration system is intended for the effective use of the heat of the exhaust gas of a gas turbine, the cogeneration system wastes the exhaust gas or the steam generated by using the heat of the exhaust gas, which reduces the overall thermal efficiency of the cogeneration system.
A gas turbine proposed in JP-A No. Hei 3-96628 uses steam for cooling its turbine nozzles to avoid the reduction of the output of the gas turbine due to the use of compressed air for cooling the stationary blades thereof. This known gas turbine supplies the steam used for cooling the stationary blades to a combustor. Consequently, it is possible that incomplete combustion occurs in the combustor if the cooling steam is supplied to the combustor at an excessively high rate. Thus part of the steam used for cooling the stationary blades must be unavoidably discharged outside, which reduces the overall thermal efficiency of the cogeneration system.
Accordingly, it is an object of the present invention to use surplus steam effectively and to cool the stationary blades of a turbine without entailing the reduction of the output and thermal efficiency of the gas turbine.
To achieve the object, the present invention provides a steam-injection type gas turbine system including an air compressor, a combustor for mixing a fuel with compressed air to burn the fuel, a turbine driven by energy of a combustion gas produced by the combustor, a waste-heat boiler using an exhaust gas discharged from the turbine as a heat source, a steam supply system for distributing steam generated by the waste-heat boiler to the combustor, stationary blades of the turbine and external steam loads, and a steam distribution adjusting means for preferentially supplying steam to the external steam loads, adjusting rate of supply of steam to the combustor and supplying the rest of the steam to the stationary blades so that the steam leaving the stationary blades are mixed in a main gas flow below the combustor. The term xe2x80x9cmain gas flowxe2x80x9d signifies the combustion gas discharged from the combustor and serving as an energy source for driving the turbine.
In the gas turbine system, the steam generated by supplying the exhaust gas discharged from the turbine to the waste-heat boiler is supplied preferentially to the external steam load, the steam is supplied to the combustor so that incomplete combustion may not occur in the combustor, and the rest of the steam is supplied to the stationary blades of the turbine to cool the stationary blades. Thus, all the generated steam can be effectively used, compressed air does not need to be used continuously for cooling the stationary blades of the turbine and hence the output and thermal efficiency of the gas turbine system are maintained high. The steam used for cooling the stationary blades of the turbine is not injected into the combustor, there is no possibility that incomplete combustion occurs in the combustor due to the supply of an excessive amount of steam into the combustor.
Preferably, each of the stationary blades of the turbine is provided with a cooling passage, and steam supplied to the stationary blades flow through the stationary blades and flow from the trailing edges of the stationary blades into the main gas flow. The steam supplied to the turbine flows through the cooling passages formed in the stationary blades efficiently cooling the stationary blades and flows from the trailing edges of the stationary blades into the main gas flow. Thus, the steam supplied to the turbine does not flow through throats between the adjacent stationary blades. Therefore the flow of the gas flowing through the throats is affected scarcely by the flow of the steam. Accordingly, the reduction of the efficiency due to the mismatched operation of the compressor and the turbine does not occur even if the flow of the steam supplied to the stationary blades of the turbine varies.
Preferably, the stationary blades of the gas turbine system to which steam is supplied are the first-stage or the second-stage nozzle blades. When steam is supplied to the first-stage nozzle blades, the thermal energy of the steam can be recovered at a very high efficiency. When the steam that has cooled the first-stage nozzle blades is discharged into the main gas flow, the steam flows through the throats between the adjacent second-stage nozzle blades. Since the flow of the steam that flows through the turbine is dependent on the sectional area of the throats, the adverse effect of the flow of the steam into the main gas flow is insignificant. When steam is supplied to the second-stage nozzle blades, thermal energy recovery percentage is slightly smaller than that at which the thermal energy of steam can be recovered when the steam is supplied to the first-stage nozzle blades, but the excessive increase of thermal load on the turbine can be avoided.
The gas turbine system according to the present invention may include further an air supply means that extracts high-pressure air from the compressor and supplies the same to the stationary blades of the turbine, and a steam/air selecting means that supplies the high-pressure air to the stationary blades when the rate of supply of steam to the stationary blades decreases.
The stationary blades of the turbine can be always effectively cooled because a reduction in steam injection rate at which steam is injected into the turbine to cool the stationary blades of the turbine is supplemented by supplying high-pressure air to the stationary blades.
Preferably, a turbine casing surrounding the stationary blades and the rotor blades of the turbine, and a main housing surrounding the turbine casing form an internal steam passage through which steam is supplied to the stationary blades of the turbine, and the turbine casing is cooled by steam that flows through the internal steam passage. Since the turbine casing is thus cooled, the tip clearance, i.e., the gap between the tip of the rotor blade and the turbine casing, may be small, so that the leakage loss of the combustion gas is small, the adiabatic efficiency of the turbine is high and the thermal efficiency of the gas turbine is high.
The steam supply system of the gas turbine system may include a NOx reducing steam jetting nozzle and an output enhancing steam jetting nozzle placed in the combustor. Steam jetted through the NOx reducing steam jetting nozzle reduces combustion temperature of the combustor to reduce NOx and steam jetted through the output enhancing steam jetting nozzle reduces the combustion temperature to enable fuel supply rate to be increased. Thus, NOx can be reduced and the output and the thermal efficiency of the turbine can be increased.
The gas turbine system may further include air adjusting mechanisms for adjusting the setting angle of the stationary blades of the compressor to adjust air flow at which air flows into the compressor, and an air control means for controlling the air adjusting mechanisms to decrease air flow according to increase in steam flow at which steam is supplied to the combustor.
The air control means monitors steam injection rate at which steam is injected into the combustor at all times and controls the air adjusting mechanisms according to steam injection rate. The air adjusting mechanisms adjust the setting angle of the movable stationary blades of the axial flow compressor to decrease air flow at which air flows into the compressor according to increase in steam injection rate. Thus, the flow of the combustion gas flowing into the turbine can be kept substantially constant irrespective of the variation of steam injection rate. Consequently, excessive increase in the pressure in the rotor chamber can be prevented, the reduction of durability of the gas turbine can be prevented, surging can be prevented, stable operation can be ensured, the reduction of the adiabatic efficiencies of the compressor and the turbine can be prevented, and the thermal efficiency of the gas turbine can be maintained on a high level. The operation of the gas turbine is moderated on the basis of a rated point corresponding to a state where steam injection rate is zero. Thus, the gas turbine system is able to operate at an efficiency at which ordinary gas turbine systems operate when steam injection rate is zero.